Since this project began, we have shown that mice exhibit body odors of exquisite individuality (odortypes) reflecting polymorphism of genes in the Major Histocompatibility Complex (MHC), and, less potently, at other autosomal sites in both sex chromosomes. Perception of differential odortypes, of which urine is the prime source, underlies mate selection and influence the success of pregnancy. The most evident fundamental biological implication of such behavioral responses to odortype perception begins with the maintenance of genetic diversity, a vital ingredient in the survival and propagation of individuals in a species. Technical progress includes investigation of odortypes by computerized, fully- automated olfactometry. In the period under review, we ascertained that commensal microorganisms. play no essential part in mouse odortype determination, found that odortype preference is due to familial imprinting, obtained decisive evidence of the involvement of different MHC genes of odortype determination, showed that HLA-transgenic mice acquire human MHC (HLA)- related odor and uncovered further instances in which the perception of non-MHC odortypes, like MHC odortypes, affects reproductive behavior. Further preliminary findings are a critical focus of current work and new aims proposed. These findings support the proposition that the urinary MHC (H-2) odortype (the kidney being a terminal differential clearing-house for responsible odorants in general) is but one of several MHC pre-renal processed fractional odortypes. These tentative data imply that secretions such as saliva and amniotic fluid exhibit tissue-distinctive MHC odortypes. Proposed on the basis of new findings is extension of study of reproducible biological functions of MHC odortypes to other familial contexts such as mother-infant and infant-infant recognition. Also proposed are systematic pursuit of the discovery that maternal MHC urinary odortypes are specifically modified by differing MHC types of fetuses in MHC-disparate mating, further investigation of imprinting to MHC types and extension of odortype work to humans.